openmp: Temporarily disable into_ssa when gimplifying OpenMP reduction clauses [PR99007]

openmp: Temporarily disable into_ssa when gimplifying OpenMP reduction clauses [PR99007]

gimplify_scan_omp_clauses was already calling gimplify_expr with false as
last argument to make sure it is not an SSA_NAME, but as the testcases show,
that is not enough, SSA_NAME temporaries created during that gimplification
can be reused too and we can't allow SSA_NAMEs to be used across OpenMP
region boundaries, as we can only firstprivatize decls.

Fixed by temporarily disabling into_ssa.

2021-02-10  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/99007
	* gimplify.c (gimplify_scan_omp_clauses): For MEM_REF on reductions,
	temporarily disable gimplify_ctxp->into_ssa around gimplify_expr
	calls.

	* g++.dg/gomp/pr99007.C: New test.
	* gcc.dg/gomp/pr99007-1.c: New test.
	* gcc.dg/gomp/pr99007-2.c: New test.
	* gcc.dg/gomp/pr99007-3.c: New test.

(cherry picked from commit bd0e37f68a3aed944df4eb739a0734bb87153749)